Calcium receptors sense extracellular Ca2+ concentration and increase intracellular Ca2+, thereby acting to suppress the production of parathyroid hormone (PTH) involved in the control of Ca2+ metabolism and bone metabolism.
The serum calcium concentration of healthy mammal is strictly maintained at about 9-10 mg/100 ml (ca. 2.5 mM), which is referred to as calcium homeostasis of living organisms. When this value falls to not more than 50%, tetania occurs, and conversely, when it increases by 50%, consciousness is clouded, both cases threatening the lives. For maintaining calcium homeostasis, duodenum acts as a Ca2+ uptake organ, bone acts as a Ca2+ storage organ, and kidney acts as a Ca2+ excretory organ. These Ca2+ kinetics are controlled by various hormones generally referred to as “calcium controlling hormone”. Representative hormone includes active vitamin D [1α, 25(OH)2D3], PTH, calcitonin, Parathyroid Hormone-Related Protein (PTH-related Protein (PTHrP)) and the like.
Bone plays an important role not only as a supporting framework and motor organ of the body, but also as a storage organ of Ca2+, which is its constituent component. To fulfill such functions, bone tissues repeat formation thereof (osteogenesis) and absorption thereof (bone resorption) throughout the entire life. For osteogenesis, osteoblast derived from mesenchymal cell plays a major role, and for bone resorption, osteoclast derived from hematopoietic cell plays a major role. The mechanism of osteogenesis includes osteoid formation by bone organic matrix (bone matrix proteins such as type I collagen and the like) produced by osteoblast present on the osteogenesis surface, and subsequent calcification. On the other hand, the mechanism of bone resorption includes adhesion of osteoclast to the bone surface, intracellular absorption of Ca2+ via protease acid secretion and ion transport, and excretion of absorbed Ca2+ to the bone marrow side, thereby releasing Ca2+ into blood. The deficient part of the bone absorbed by osteoclast is repaired by osteogenesis by osteoblast. This series of phenomena are called remodeling of bone, and by the remodeling, old bones are replaced by new bones, thus maintaining the strength of the entire bone while maintaining calcium homeostasis.
When blood Ca2+ concentration increases, calcium receptor senses it, immediately suppresses secretion of PTH from the parathyroid gland to decrease the amount of Ca2+ to be supplied into the blood [Brown, E. M., Homeostatic mechanisms regulating extracellular and intracellular calcium metabolism, in the parathyroids, p. 19, (1994), Raven press, New York]. Secretion of PTH is also suppressed by active vitamin D [1α, 25(OH)2D3].
Because PTH is a hormone assuming an important role in controlling Ca2+ metabolism and bone metabolism, attempts have been made to apply PTH to the treatment of osteoporosis. In 1982, Tam et al. found that sustained administration of bovine PTH (1-84) to thyroid/parathyroid gland enucleated rat results in promotion of both osteogenesis and bone resorption of femoral cancellous bone, leading to a decrease in net bone mass, but subcutaneous intermittent administration thereof does not result in promotion of bone resorption but in promotion of osteogenesis alone, leading to an increase in the bone mass [Endocrinology, 110, 506-512 (1982)]. Furthermore, Uzawa et al. compared the actions of sustained administration and intermittent administration of PTH with regard to epiphyseal long bone and metaphyseal cancellous bone of young rat. As a result, they clarified that sustained administration of PTH results in remarkable increase in bone mass in metaphyseal cancellous bone highly susceptible to the effect of enchondral ossification, though associated with abnormal findings such as hyperplasia of epiphyseal plate cartilage, fibrous ostitis and the like, and in marked promotion of bone resorption and decrease in bone mass accompanied by rarefaction of cortical bone, in epiphyseal cancellous bone where the effect is small [Bone, 16, 477-484 (1995)]. In addition, it has been reported that intermittent administration of PTH results in significant increases in bone mass and bone trabecula in both epiphyseal and metaphyseal cancellous bones without increase in osteoclast or decrease of cortical bone.
Moreover, Scutt et al. have reported that, in chicken calvaria derived osteoblast, a short time (10-20 min) treatment with PTH promotes cell growth as compared to a long time (18 hr) treatment [Calcif. Tissue Int., 55, 208-215 (1994)]. This suggests that some of the actions of PTH on osteoblast are temporary and that expression of the action by the treatment for an extremely short time may be related to the fact that sustained administration and intermittent administration of PTH in vivo show different actions on bone tissues.
Ishizuya et al. further clarified through investigation of the action of PTH on differentiation of osteoblast using an in vitro experiment system that the action of PTH varies depending on the treatment time. They have reported that sustained action of PTH on osteoblast derived from rat calvaria resulted in strong inhibition of differentiation of osteoblast and nearly complete inhibition of osteogenesis in vitro, but repeated PTH action for the first 6 hr of 48 hr as one cycle resulted in significant promotion of differentiation of osteoblast and promotion of osteogenesis in vitro.
PTH is considered to not only prevent decrease in bone mass of osteoporosis model, but also has a bone mass recovery effect even on an animal already suffering from marked decrease in bone mass. Wronski et al. intermittently administered human PTH (1-34) to 90-day-old SD rat at 4 weeks post-ovariectomy and showing an obvious decrease in cancellous bone, for 15 weeks from 4 weeks post-ovariectomy. As a result, promotion of osteogenesis and inhibition of bone resorption were observed during the period of from week 5 to week 10 after the start of the administration, showing increased bone mass of about twice the bone mass of sham operation group [Endocrinology, 132, 823-831 (1993)]. They have also reported that, in this experiment, estrogen and bisphosphonate prevented decrease in bone mass caused by ovariectomy but did not show increase in bone mass, unlike PTH. They analyzed in detail the cortical bone of this experiment system and found images showing promoted osteogenesis and bone mass increase on the periost side and endosteum side by intermittent administration of human PTH (1-34), based on which they have clarified that the increase in cancellous bone due to PTH did not accompany decrease in cortical bone [Bone, 15, 51-58 (1994)].
Furthermore, Mosekilde et al. have reported that intermittent administration of human PTH (1-34) or human PTH (1-84) causes not only an increase in bone mass but also a dose-dependent increase in compression strength and bending strength, which are indices of bone substance, of cancellous bone [Endocrinology, 129, 421-428 (1991)] and cortical bone [J. Bone Miner. Res., 8, 1097-1101 (1993)] of rat vertebral bone. As discussed above, since PTH shows an obvious bone mass increasing action in experimental animals, various investigations are ongoing as regards the restrictive conditions expected in actual clinical applications. Mizoguchi studied whether or not a pharmacological effect is observed by intermittent administration of PTH, even when PTH in blood, which is considered to be one of the factors responsible for osteoporosis, has significantly increased, and concluded that the bone mass increased as usual [Journal of Japanese Society of Bone Morphometry, vol. 5, pp. 33-39 (1995)]. Takao et al. have studied the frequency of PTH administration and reported that administration of once a week for 12 weeks to healthy rat scarcely promoted bone resorption but dose-dependently increased the bone mass [Japanese Journal of Bone Metabolism, vol. 12 (Suppl.), p. S343 (1994)], suggesting possible effectiveness of clinically useful low frequency administration. The foregoing achievements suggest the possibility of PTH for making a potent and promising therapeutic drug for the treatment of postmenopausal osteoporosis or postovariectomy osteoporosis, which increases bone mass and decreases bone fracture rate.
These results clearly indicate that intermittent administration of PTH would enable treatment of osteoporosis. On the other hand, PTH problematically requires injection as an administration route, which is painful for many patients. However, an orally administrable pharmaceutical agent that can intermittently increase PTH concentration in blood is greatly expected to become a therapeutic drug of osteoporosis, which is based on a new action mechanism different from that of the above-mentioned PTH and conventional calcitonin.
Calcium receptor is a G protein coupled receptor, which is cloned as a molecule essential for controlling PTH secretion, and which penetrates cell membrane 7 times. Human calcium receptor consists of 1078 amino acids, and shows 93% amino acid homology with bovine calcium receptor. Human calcium receptor consists of a large N terminal extracellular region consisting of 612 amino acids, a cell membrane penetration region consisting of 250 amino acids and a C terminal intracellular region consisting of 216 amino acids.
Expression of calcium receptor has been found in parathyroid gland, kidney, thyroid C cell, brain and the like, as well as in bone (bone marrow cells).
When calcium receptor is bound with a ligand such as Ca2+ and the like, it activates phospholipase C in conjugation with G protein, causes production of inositol triphosphate and increase in intracellular Ca2+ concentration, and as a result, suppresses secretion of PTH [Nature, 366, 575-580 (1993)].
As mentioned above, a pharmaceutical agent that inhibits activation of calcium receptor, or a pharmaceutical agent that antagonizes calcium receptor, removes suppression of PTH secretion in parathyroid gland cells, and promotes secretion of PTH. It is considered that, if the antagonistic action can increase blood PTH concentration discontinuously and intermittently, its antagonist is expected to show the same effect as that provided by intermittent administration of PTH, and a pharmaceutical agent extremely effective for the treatment of osteoporosis can be provided.
In contrast, cytochrome (cytochrome P450, hereinafter P450) is a protein having a molecular weight of about 50,000, which contains protoheme, and its physical functions vary over a wide range. For example, it has a function of an enzyme catalyzing various reactions in the drug metabolism. CYP2D6 belonging to the family of P450 (CYP) is an important enzyme for human drug metabolism, and is involved in the metabolism of many compounds. When a drug inhibiting the metabolic function of CYP2D6 is administered, the drug is accumulated in the body and may exert a strong influence. Accordingly, a compound having a weak inhibitory action on the metabolic function of CYP2D6 is desirable as a drug.
Heretofore, various compounds useful as CaSR antagonists have been reported.
Specifically, for example, a compound represented by the following formula
wherein A is aryl etc., D is C or N, X1 and X5 are each hydrogen, cyano etc., and X2, X3 and X4 are each hydrogen, halogen, C1-4 alkyl etc. (WO 02/38106) is mentioned.
In addition, a compound represented by the following formula
wherein A is aryl etc., D is C or N, X1 and X5 are each hydrogen, cyano etc., X2 is hydrogen etc., and X3 and X4 are each hydrogen, C1-4 alkyl etc. (WO 02/34204) is mentioned.
Furthermore, a compound represented by the following formula
wherein A is C or N, D is C or N, X is cyano, nitro etc., Y is chlorine, fluorine etc., and Ar is phenyl, naphthyl etc. (WO 02/07673) is mentioned.
In addition, a compound represented by the following formula
wherein X is cyano, nitro etc., Y is chlorine, fluorine etc., and Ar is phenyl, naphthyl etc. (JP 2002-536330-T, WO 00/45816, EP 1148876-A, U.S. Pat. No. 6,417,215), and a compound represented by the following formula
wherein X1, X2, X3, X4 and X5 are each H, halogen and the like, Y1 is a covalent bond, or a non-substituted or a substituted alkylene, etc., Y2 is a non-substituted or a substituted methylene, C1-4 alkyl etc., Y3 is a covalent bond, O etc., R3 and R4 are each independently methyl, ethyl etc., R5 is aryl, fused aryl etc., R7 is H, OH etc., R8 is H, C1-4 alkyl etc., A and B are independently a bond, CH2 etc., G is a covalent bond, CHR6 (R6 is H etc.) etc. (JP 2002-510671-T, WO 99/51569, EP 1070048-A, U.S. Pat. No. 6,395,919) are described.
As a CaSR antagonist, a compound represented by the following formula
wherein X is a compound represented by the following formula
wherein X1, X2, X3 and X4 are each independently CN, NO2 etc., then W is R1, SO2R1 etc., R2 is H, C1-4 alkyl etc., and the like, Y1 is a covalent bond, or a non-substituted or a substituted alkylene etc., Y2 is a non-substituted or a substituted methylene etc., Y3 is a covalent bond, O etc., R3 and R4 are independently methyl, ethyl etc., R5 is heteroaryl, fused heteroaryl etc., R7 is H, OH etc., R8 is H, C1-4 alkyl etc., A and B are each independently a bond, CH2 etc., and G is a covalent bond, CHR6 (R6 is H etc.) etc. (JP 2002-510636-T, WO 99/51241, EP 1069901-A, US 2002052509-A) is described.
As a CaSR antagonist, a compound represented by the following formula
wherein Y1 is a covalent bond, alkylene etc., Y2 is a non-substituted or a substituted methylene, C1-4 alkyl etc., Z is a covalent bond, O etc., R3 and R4 are each independently methyl, ethyl etc., R5 is phenyl, naphthyl etc., G is a covalent bond or C—R6 wherein R6 is H, OH etc., R7 is H, OH etc., R8 is H, C1-4 alkyl etc., A-B moiety is CH2CH2, a covalent bond etc., and X is a following formula
wherein W is R1, SO2R1 wherein R1 is hydrogen, C1-4 alkyl etc., and the like, X1, X2, X3 and X4 are each independently CN, NO2 etc., R2 is hydrogen, C1-4 alkyl etc., and the like (JP 2001-523223-T, WO 98/45255, EP 973730-A, U.S. Pat. No. 6,294,531), a compound represented by the following formula
wherein R1 is aryl etc., R2 is hydroxyl group etc., R3 and R4 are each lower alkyl etc., R5 is substituted naphthyl, substituted phenyl etc., Y1 is alkylene etc., Y2 is alkylene, Y3 is alkylene and Z is oxygen etc. (JP 2001-501584-T, WO 97/37967, EP 901459-A, U.S. Pat. No. 6,022,894), a compound represented by the following formula
wherein X is nitro etc., Y is hydrogen etc., Q is C1-4 alkyl etc., Ar is phenyl, naphthyl etc., m is 0-2 and n is 1-3 (JP 2002-522499-T, WO 00/09132, EP 1112073-A), and a compound represented by the following formula
wherein X is cyano etc., Y is chlorine etc., Q is hydrogen etc., W is oxygen etc., D is hydrogen etc. and n is 2-4 (JP 2002-522532-T, WO 00/09491, EP 1104411-A) are described.
Maxine Gowen et al. administered a compound having a CaSR antagonistic action, which is called NPS-2143,
to OVX rats orally and measured blood concentration and bone density thereof, thereby testing the effect of NPS-2143 on osteogenesis, and reported the results thereof (The Journal of Clinical Investigation, vol. 105, pp. 1595-1604 (2000)).
According to the report, NPS-2143 significantly promotes release of PTH, but it did not show any direct effect on osteoblast and osteoclast in vitro and was free of bone decrease or bone increase. One of the reasons pointed out therefor is too long a half-life of NPS-2143 in blood. That is, when rat PTH (1-34) was administered to OVX rat at the dose of 5 μg/kg, blood PTH concentration reached the peak of about 175 pg/ml in 30 minutes and returned to the original level in 2 hours, but when NPS-2143 was administered at the dose of 100 μmol/kg, the blood PTH concentration reached about 115 pg/ml in 30 minutes and kept increasing and showed about 140 pg/ml even 4 hours later (The Journal of Clinical Investigation, vol. 105, p.1595-1604 (2000), especially p. 1598, FIG. 3).
At that time, the blood concentration of NPS-2143 itself was maintained at the level of not less than 100 ng/ml even 8 hours after the administration. It was 24 hours later when the concentration became 10 ng/ml or below the undetectable level.
The above-mentioned Maxine Gowen et al. reference teaches that a calcium receptor antagonist having a too long blood half-life provides results as in continuous administration of PTH, where a bone mass increase cannot be expected. Thus, most of the conventional calcium receptor antagonists continuously increase the blood PTH concentration and cannot be expected to provide a sufficient osteogenesis promoting action. Of the conventional calcium receptor antagonists, a compound represented by the following formula [I]
wherein R1 is optionally substituted aryl group etc., R2 is C1-6 alkyl group, C3-7 cycloalkyl group etc., R3 is hydroxyl group etc., R4 is hydrogen atom etc., R5 and R6 are C1-6 alkyl group etc., R7 is optionally substituted aryl group etc., X1 is a single bond, C1-6 alkylene etc., X2 is optionally substituted C1-6 alkylene, X3 is a single bond or optionally substituted C1-6 alkylene, and X4 and X5 are linked to form a single bond, methylene etc., which has a superior calcium receptor antagonistic action, which can be administered orally and intermittently, and which can increase blood PTH concentration discontinuously and intermittently, is disclosed (WO02/14259). By comparison of the activities of a compound within the scope disclosed in this publication and the compound of the present invention, the compound of the present invention was surprisingly found to have a higher activity and to be a compound having a lower inhibitory action on the metabolic enzyme CYP2D6.
However, there are not many reports on such an effective compound, and further study is desired.